The present invention relates to a method and apparatus for performing microwave plasma anisotropic dry etching. More particularly, it relates to a microwave plasma processing apparatus having an improved anisotropic etching capability and processing rate for etching or ashing integrated circuit semiconductor (IC) substrates.
In fabrication processes for forming fine patterns in IC devices, etching processes are becoming increasingly important. From the beginning of the IC industry, "wet" or chemical etching methods have been employed, but recently "dry" etching methods, such as plasma etching methods, have been adopted. Dry etching processes have various advantages, as compared with other etching methods including, for example, the capability to provide high resolution, less under-cutting, the elimination of fabricating steps such as wafer handling for rinsing and drying, and inherent cleanliness. Importantly, plasma etching makes it possible to obtain extremely fine pattern resolution on the sub-micron order and to perform sequential etching and stripping operations using the same machine, thus making it possible to have a fully automated process for fabricating an IC.
A plasma is a highly ionized gas containing a nearly equal number of positively and negatively charged particles (electrons and positive or negative ions) plus free radicals. The ions are utilized for etching in a sputtering process, which is essentially a physical etching, and the free radicals are utilized for chemical etching. Free radicals are chemically activated, electrically neutral atoms or molecules which can actively form chemical bonds when in contact with other materials, and are utilized in a plasma etching process as a reactive species which chemically combines with materials to be etched. The gas and the material to be etched are selected so that the free radicals combine with the material to form a volatile compound which is removed from the system by an evacuating device. Consequently, an etching method utilizing free radicals is essentially chemical etching.
Usually, a plasma etching apparatus comprises a plasma generating device, a reacting region (reaction chamber) and an exhausting device.
The basic requirements for plasma etching of an IC substrate are a high etching rate, anisotropic etching accompanied by reduced side etching, and less damage to the substrate due to ion or electron bombardment, radiation by ultraviolet rays, and the like. Recently, in accordance with a demand for high density packaging and high speed signal processing capability of ICs, it has become necessary to form increasingly fine and precise etching patterns on IC substrates. For example, so-called micro-patterns--e.g., wiring patterns having sub-micron dimensions, such as a width of 0.5 to 0.8 .mu.m--are required. To achieve such accuracy for precise micro-patterns, a plasma etching method and apparatus having the capability to perform highly anisotropic etching provided by a high controllability of the directions of radicals and ions, is considered a key technology. With an anisotropic etching method, functional particles, namely ions and free radicals, must be controlled to impinge substantially perpendicularly on the surface of an IC substrate, which is covered with a patterned lithographic mask of photoresist material or silicon nitride. Since the etching process is carried out perpendicularly to the exposed surface of the substrate, side etching or undercutting of the patterned layer is reduced and the etched pattern has dimensions which are sufficiently accurate to form fine patterns of a sub-micron order with high reproducibility.
The advantages and disadvantages of conventional technologies are surveyed briefly. With an ion etching process, ions generated in a plasma are accelerated by an electric field and impact the exposed surface of a substrate to be processed, thereby to remove the material of the substrate by their kinetic energy. The electric field is usually applied by an external electric source. The direction of movement of the ions is controlled by the applied electric field, resulting in anisotropic etching regardless of the gas pressure. In addition, a high etching rate is obtained with a high accelerating potential and high gas pressure. However, ions of relatively high energy tend to damage portions of the substrate near the portions of the substrate undergoing etching. For example, photoresist material used as a mask may be carbonized by the ion bombardment. Further, fine wiring conductors formed of doped polysilicon may be damaged, and metal members of the etching apparatus which are bombarded by the ions are sputtered onto the substrate undergoing etching, thereby contaminating the substrate. These damaging effects can be eliminated by the use of a low accelerating energy.
When a low accelerating energy is used for ion etching the substrate is mounted in and electrically isolated from the apparatus, and exposed to the plasma. Thus, the substrate is electrically floated and negatively charged with respect to the plasma, resulting in the formation of an ion sheath in front of the substrate undergoing processing. The ion sheath is created because of the difference between the velocity of electrons and that of ions in the plasma. A potential drop, referred to as a floating potential, is generated across the ion sheath--the floating potential being approximately 20 V. However, with low accelerating energy etching, the etching rate is reduced.
On the other hand, plasma etching utilizing free radicals is entirely free from the disadvantages and damages caused by sputtering. Unfortunately, the direction in which the radicals impact the substrate is not controllable by an electric field because the radicals are not electrically charged. When plasma etching is conducted in a relatively high gas pressure such as 10 to 10.sup.-1 Torr, the mean free path of the gas molecules is short, e.g., 5.times.10.sup.-2 to 5.times.10.sup.-4 cm, resulting in multiple collisions of the molecules and causing the molecules to move in random directions. Chemical etching (or so-called "after glow etching") using a high gas pressure is not suitable for anisotropic etching because the free radicals have random motion and do not strike the substrate perpendicularly.
In order to eliminate the random motion of the radicals and to conduct satisfactory anisotropic etching, a longer mean free path of the gas molecules, far exceeding the dimensions of the etching apparatus, is necessary. A longer mean free path can be provided by a lower gas pressure, e.g., approximately 10.sup.-4 Torr. In this case, the mean free path of the gas molecules is relatively long, approximately 50 cm, in comparison with the corresponding dimension of the etching device. If the initial direction of the radicals is limited by a geometrical shielding structure similar to an aperture or shield, the radicals ejected from a plasma in a given direction continue in that direction until they impact the substrate. Therefore, the shielding structure is designed to select radicals having a favored initial direction. However, a reduction of the etching rate is inevitable since the number of free radicals which impact the substrate is lowered. In order to clarify the problems described above, some typical prior art etching apparatus and technologies are described in the following.
FIG. 1 is a schematic cross-sectional view of a reactive ion etching (RIE) apparatus, illustrating the structure and the fundamental operating principle of a conventional RIE apparatus. A pair of electrodes 2 and 3 are arranged in parallel in a vacuum tight vessel 12. The vessel is evacuated by an exhausting pump system (not shown) through an exhausting tube 14, as indicated by an arrow EX, and gas is fed to the vessel through a feeding tube 13. Workpieces (IC substrates) 1 to be etched are mounted on a quartz tray 7 arranged on the electrode 2 and radio frequency power of 13.75 GHz frequency is applied to the electrode 2, which is electrically isolated from ground. Thus, a plasma discharge 5 is generated between electrodes 2 and 3. Since the workpieces or substrates 1 are isolated by an insulator 9, a cathode wall or ion sheath 6 is formed in front of the electrode 2. Ions 4, generated in the plasma 5, are accelerated by an electric field generated within the ion sheath 6, impinge the substrates 1 with an energy of approximately 100 eV, and etch the surfaces of the substrates 1. A ground shield 8 shields the electrode 2 to stabilize the plasma. The ions are guided by the electric field inside the ion sheath 6 to impact the substrate 1 substantially perpendicularly to the surface, thus realizing substantially anisotropic etching. However, damage due to ion sputtering is inevitable.
In another plasma etching method utilizing plasma ions, referred to as a reactive ion beam etching (RIBE), ions are extracted from a plasma by a strong static electric field to impact the substrates to be etched. A plasma etching apparatus referred to as an "ion shower," proposed by Matsuo et al., is a typical RIBE apparatus and is disclosed in Japanese laid open patent publication No. 55-141729. However, the energy of the ions in the ion shower is approximately 500 to 1000 eV, which causes sputtering damage, as discussed above.
In order to overcome the problems associated with sputtering, various apparatus have been proposed, such as a chemical dry etching (CDE) apparatus disclosed in U.S. Pat. No. 4,192,1980--Horiike et al. In CDE apparatus, gas pressures as high as 10 to 10.sup.-1 Torr and the presence of neutral radicals moving in random directions result in isotropic plasma etching which fails to form precise and fine micro-etching patterns.
A rather low gas pressure of approximately 10.sup.-3 to 10.sup.-4 Torr is employed in certain anisotropic plasma etching apparatus. In one such anisotropic plasma etching apparatus, reported in an article entitled "Low Energy Ion Beam Etching," by H. R. Kaufman et al., J. Electrochem. Soc., Vol. 128, No. 5, May 1981, the substrates are mounted on an insulator which is electrically isolated from the system, resulting in the creation of an ion sheath having a floating potential with a voltage drop of approximately 20 V; the ions generated in the plasma are accelerated by the relatively small floating potential of the ion sheath, resulting in little damage to micro-patterns formed on the IC substrate. Further, the gas pressure is kept rather low--approximately 0.8 to 4.times.10.sup.-4 Torr. The thickness of the ion sheath formed in front of the substrate is much smaller than the mean free path of the ions in the plasma, and thus the ions move through the ion sheath without colliding with other ions or molecules. Since the ions are guided by the electric field inside the ion sheath, almost all the ions strike the workpiece at an angle which is normal to the surface of the workpiece. Accordingly, this plasma etching method is effective to obtain precise micro-patterns. The etching is considered to be performed by a combination of the radicals and ions, which is referred to by the author as an "enhanced chemical etching". However, since the floating potential is utilized to accelerate the ions, the etching rate is very small and thus this apparatus is not suitable for practical use.
Another etching apparatus utilizing low energy ions is reported by Suzuki et al. in an article titled "The roles of Ions and Neutral Active Species in Microwave Plasma Etching," J. Electrochem. Soc. Vol. 126, No. 6, June 1979. The apparatus is referred to as a "magneto-microwave plasma etching apparatus" by the author, because the apparatus has a plasma discharging chamber disposed in a magnetic field. A 2.45 GHz microwave is fed through a wave guide to the plasma chamber, and a magnetic field is generated in the plasma chamber using solenoids and a permanent magnet. A low pressure gas, such as CF.sub.4 at 5.times.10.sup.-4 Torr, is fed into the plasma chamber and the microwave energy causes the gas to discharge, generating a plasma. The magnetic field is utilized to drive the electrons in the plasma in a Raman motion (cyclotron motion), increasing the number of collisions in the gas molecules to achieve a higher ionization efficiency. Thus, even with a low gas pressure, a sufficient number of ions are obtained. The magnetic field is a mirror-type magnetic field having a peak magnet field intensity, namely, a mirror peak, located at the opposite side of the plasma from a substrate to be etched. The mirror peak serves to confine the plasma and faces in the direction of propogation of the microwave energy. The magnetic mirror, however, does not reflect or suppress electrons and thus fails to prevent the electrons in the plasma from striking the substrate which is electrically floated and exposed to the plasma. Accordingly, plasma screening equipment is provided to restrict exposure of the substrate to the plasma. The screening equipment includes four tungsten mask electrodes. The first electrode (ground potential) determines the plasma potential; the second electrode extracts ions from the plasma and suppresses electrons; the third electrode suppresses ions; and the fourth electrode has the same electrical potential as the substrate. The accelerating energy of the ions is approximately 20 V, as described with respect to the Kaufman et al. apparatus, and etching is performed in an anisotropic manner. However, the screening equipment reduces the etching rate and since the plasma screening equipment is extremely close to the substrate, the electrons generated in the plasma can travel to a region in the plasma which is close to the substrate. These electrons collide with neutral, non-ionized gas molecules (not radicals), ionize the neutral molecules and create additive radicals which travel in random directions causing side etching and reducing the anisotropic etching capability of the apparatus. In addition, when the electrons from the plasma impinge onto the substrates, the kinetic energy of the electrons is converted into heat, elevating the temperature of the substrate. If the substrate to be etched is covered with a photoresist layer, a problem is introduced, in that the temperature elevation of the substrate softens the photoresist mask and reduces the dimensional accuracy of the etching.
Another example of a low energy plasma etching apparatus, developed by Akitani, is reported in the publication titled "Directional Dry Etching of Silicon by a Reactive Nozzle-Jet" presented at the Third Dry Process Symposium, held on Oct. 26-27, 1981, in Tokyo. In this apparatus, a free jet of radicals generated at a gas pressure of approximately 0.1 to 1.0 Torr is ejected from an aerodynamic nozzle system. A free-expansion process occurs as the radicals pass from the nozzle to a vacuum at a pressure of approximately 10.sup.-4 to 10.sup.-5 Torr. The free-expansion process collimates the direction of the motion of the radicals, which initially have random motion. The collimated direction of the radicals enables the radicals to impinge on the surface of the substrate with a low energy a nearly normal direction. Although this apparatus forms satisfactory precise micro-patterns on a substrate, the etching rate is very small, due to the extremely small nozzle diameter; thus, the apparatus is not suitable for practical use.
Accordingly, it has been necessary to develop a substantially anisotropic microwave plasma etching apparatus having a low energy ion etching system and a reasonable etching rate, and which is suitable for practical use.